Physical behaviour of glycolipids in bilayer membranes: distribution and accessibility.

A 3-dimensional scale drawing has been produced which purports to illustrate key features of glycolipid environment in the eukaryote plasma membrane. An attempt has been made to isolate two such features for separate study using bilayer model membranes. In particular, i) physical shielding of glycolipid headgroups by membrane-associated macromolecules has been investigated as a possible source of glycolipid crypticity; and ii) freeze-etch EM has been employed in an attempt to understand factors that control glycolipid receptor function and distribution. The first of these problems was approached by measuring sialic acid released (by the enzyme, neuraminidase) from gangliosides in lipid bilayers. The bilayers could be coated with a firmly attached layer of protein or high molecular weight polysaccharide in order to mimic the presence of macromolecules at a cell surface. In fact, surface coat material did not measurably reduce sialic acid release from gangliosides in fluid or rigid phosphatidylcholine membranes. The implication drawn is that non-specific shielding by surface glycoproteins is an unlikely source of glycolipid crypticity in cell membranes. The second problem was attacked by direct visualization of lectins and bilayer vesicles on the platinum-shadowed outer surface of large liposomes. To a first approximation bound lectins (and hence gangliosides) were equally distributed between coexisting fluid and rigid phospholipid domains. However the appearance was suggestive of a tendency for gangliosides to exist in small clusters. Lipid vesicles bearing lectin receptors were used to mark points of lectin-mediated membrane-membrane attachment on large liposomes. There was no obvious difference in the extent of such attachment to fluid and rigid domains.